There are known injection molding devices of the general type above-referred to which comprise a heatable pressure mold and a cylindrically shaped housing in which a screw extruder rotates to plasticize preliminarily plasticized material. The plasticized material is fed to a distributor duct which together with a cylinder-piston system constitutes an injection unit. This injection unit communicates with the pressure mold via a nozzle and is operated by means of a cylinder-piston servo system and a valve head which constitute a ventil device that is alternately connected to the plasticizing means or the injection unit.
In the manufacture of rubber articles, a temperature gradient occurs unavoidably between the heated pressure mold and the piston-operated injection means. The presence of such a sharp temperature gradient causes considerable manufacturing difficulties.
The vulcanization of the injected material occurs in the pressure mold and such vulcanization requires a much higher temperature than is needed during the plasticizing and the injection operation proper. The result is that there is a considerable conduction of heat from the mold to the injection nozzle and thus to residual plasticized material still present in the injection nozzle. As a result of the heat conduction, a temperature exists within the range of the injection nozzle such that material remaining in the injection nozzle is partly vulcanized and such partly vulcanized material will be forced into the mold during the next following injection cycle.
The presence of partly vulcanized material and of fresh plasticized material which is forced into the mold during the next following injection operation may cause flaws in the bodies to be formed in the mold, and, thus, a general decline in the quality of the obtained products.
The same disadvantageous results occur when the molded bodies consisting of heat-setting reaction masses--for instance--Duroplasten--are produced.
During the setting of such material in the pressure mold, a setting of residual material to be injected occurs in the injection nozzle and such residual material is forced during a subsequent injection cycle into the mold together with fresh material and this also results in flaws in the molded bodies.
It is known for instance, from DT-AS No. 1,197,610 (published German Patent application) to provide injection molding devices in which it is attempted to limit the damaging temperature levels of reaction masses so that the material to be pressed is exposed to the injection temperature only for a very short period of time or is held at the reaction temperature. Moreover, the injection device proper is so built that it can be very easily disassembled into the components to be cleaned. Such disassemblly and subsequent cleaning, however, easy, still entails stopping of the entire installation for a certain length of time.
The duration of the effect of heat at the reaction temperature is also essentially influenced by the shape and size of the molded body to be produced. Hence, the arrangement as disclosed in the German publication can be used only under limited conditions and, in particular, a reaction of the reaction mass at the nozzle can not be prevented.
Moreover, in the known valve device after opening and closing of the distributor channel communicating with the injection nozzle, a residue of the material remains after each injection operation and such residue is likely to set.